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Trouble inputing data to an Array

Hello all.
New programming novice here that has run into a weird issue.

Trying to create a program to input student data from a text file into an array, but getting an error when I run it. I am probably doing something wrong in a very simple way, but just not getting it.

I can run the sample program one of two ways, as input into a single class, or into an array. It works and returns the data correctly as a single class, but when I try to do the array part, it compiles okay, but throws an error on running: NullPointerException

import java.io.*;

import javax.swing.*;



public class GradeReader

{

        final int NUM_PERSON = 4;

        Grades[] data = new Grades[NUM_PERSON];

        // Grades data = new Grades();

        

        String firstName, lastName, classNum, grade;

        

        public static void main(String[] args) throws IOException

        {

                new GradeReader();

        }                

        

        public GradeReader() throws IOException  

        {

                getFile();                 

                JOptionPane.showMessageDialog(null, "Reading data from File. Please hit 'OK' to continue.");        

                showResults();        

        }        

        

        private void getFile() throws IOException

        {

                String filename = "Grades.txt";

                FileReader freader = new FileReader(filename);

                BufferedReader inputFile = new BufferedReader(freader);



                for (int index = 0; index<3; index++)

                        {

                                firstName = inputFile.readLine();

                                lastName = inputFile.readLine();

                                classNum = inputFile.readLine();

                                grade = inputFile.readLine();

                                

                                data[index].setGrades(firstName, lastName, classNum, grade);

                                // data.setGrades(firstName, lastName, classNum, grade);

                                

                        }

                inputFile.close();

        }

        

        private void showResults()

                {

                        JTextArea outArea;

                        JScrollPane scroller;

                        outArea = new JTextArea(30,35);

                        outArea.setText(data.toString());

                        scroller = new JScrollPane(outArea);

                        JOptionPane.showMessageDialog(null,scroller);

                        System.exit(0);

                }        



}

Let me know if you want to see any of the other code, or the txt list that the input is coming from.

Grades[] data = new Grades[NUM_PERSON];

This code creates an array capable of storing NUM_PERSON Grades objects. By default, each index is initialized to null.

data[index].setGrades(firstName, lastName, classNum, grade);

You are attempting to call the method setGrades on null, hence the NullPointerException.

One solution is to initialize each index of data to a new instance of Grades in the GradeReader class' constructor, which will be called when a new instance of GradeReader is created:

public class GradeReader {



      public GradeReader() {

            for (int i = 0; i < NUM_PERSON; i++)

                  data[i] = new Grades();

      }



...

Okay, that got me past the NullPointer error. But I am not getting my output now.

Here is the output from my Grades class:

        

public String toString()

                {

                        String studentStr = "Student name: " + getFirstName() 

                        + " " + getLastName()

                        + "\nClass: " + getClassNum() 

                        + "\nGrade: " + getGrade();

                        

                        return studentStr;

                }

And here is where I am trying to call it:

private void showResults()

                {

                        JTextArea outArea;

                        JScrollPane scroller;

                        outArea = new JTextArea(30,35);

                        outArea.setText(data.toString());

                        scroller = new JScrollPane(outArea);

                        JOptionPane.showMessageDialog(null,scroller);

                        System.exit(0);

                }

Again, I got the output doing a single object, but now that I do have mutliple objects in the array, I get junk returned.

I assume I need to do something with this line:
outArea.setText(data.toString());
But so far I have not been able to come up with the right option.

Thank you.

Well, I partly figured it out.

When I do this:

outArea.setText(data[0].toString() + data[1].toString());

I get my output. But it seems to be that there must be a much better way of doing this.

If you want to print every item in the list, use a loop. Consider cocatenating each item into a temporary String variable, or using JTextArea's append method.